What if the medium were a waste material - would the P.G. still have standing to certify documents as it related to the treatment and disposal of that material?
In California, waste management issues at factories, landfills, municipal water treatment plants, etc. are normally handled by PEs. But if wastes from these operations affected soil or groundwater, then a PG could enter the picture. In California, PGs commonly manage waste soil or wastewater generated during environmental remediation projects.
There are limits to what a PG can do. For example, PGs often oversee all aspects of routine remedial excavations, including confirmatory sampling, treatment or disposal of waste soil, and preparation of closure reports. But if a remedial excavation required a grading or shoring plan, or threatened a neighboring structure, then input from a Civil (or Geotechnical) PE would be required. As another example, PGs often prepare monitoring reports for sites with groundwater treatment systems, including summaries of routine treatment system operation. However, a Civil PE would be needed for the permitting, design, installation, or modification of the treatment system .
Note that California probably empowers PGs to a greater degree than most other states. California has a relatively long history of geologist licensure; geologists were first licensed at the local level in the 1950s, and have been licensed at the state level since about 1970. Also, PGs in California commonly take more exams than the two 4-hour ASBOG FG/PG exams required in most other states. I've taken 16 hours of geology exams in California myself, and it's theoretically possible to take as many as 25 hours for all of the available certifications in general geology, engineering geology, hydrogeology, and geophysics.
California is a geologically active state, and things like mining, oil production, and water supply are all major economic issues. Because of these factors, I suspect that geologists here may have a higher professional profile and status than they do in other states.